A talk from our Secretary, Barry Hayden, on the subject “Time”,
given at the Club AGM on 13th May 2024.
Barry’s talk considered the history of time, from our agricultural past to the current need to define time as accurately as possible.
Our initial concept of time was taken from the Sun, the seasons that were determined from our experience, and our daily tasks varied with the seasons.
What we call “time” is our measure of the passing of time, and is primarily a solar calendar.
Around 330BC, a Greek student determined the length of the tropical year to be 365¼ days. He also showed that the four seasons had different lengths, which was maybe the first realization of what we now understand as the eccentricity of the orbit of the Earth around the Sun.
The Romans invented the 365 day calendar by trial and error. At the time, there were only ten months in the calendar, while there are just over 12 lunar cycles in a year. Julius Caesar’s astronomers explained the need for 12 months in a year and the addition of a leap year to synchronise with the seasons.
In the Islamic calendar the new crescent Moon marks the start of a new month. It takes 29.5 days for the moon to go through all of its phases, but it isn’t practical for a month to have half a day. An Islamic month can therefore have either 29 or 30 days.
Our current calendar is the Gregorian calendar dating from 1582.
The ancient Egyptians were the first to use 24 hours to divide a day, dividing daytime into 12 hours and night into a further 12 hours. The Egyptians used sundials and water clocks to track time, but there is little evidence of control less than one hour.
The UK adopted sundials. They were common in town squares, and can still be seen. Sundials were replaced with pendulum clocks, frequently chiming “on the hour”.
The Sun rises in the East and sets in the West, the difference in sun-time across England being approx. 20 minutes. Each town had its own town clock which was set to local noon when the sun was at its highest. When travel started between towns, people reset their watches at each town.
Following the industrial revolution it became important to set a standard time. With the railways matters moved at a fast pace, which led to the spread of “railway time”. Greenwich Mean Time was ultimately adopted Great Britain by the Railway Clearing House in December 1847, by the mid-1850’s almost all public clocks were set to GMT, and it finally became Britain’s legal standard time in 1880.
GMT was established in 1884 at the International Meridian Conference, to establish a standardized system for measuring world longitude. They decided that the meridian passing through the Royal Observatory in Greenwich would be 0o longitude.
There was a need for more accurate clocks/watches, in particular for shipping and the determination of longitude. The pendulum clock was invented in 1656, but the motion of a ship prevented a pendulum from keeping time at sea. The problem was resolved by an engineer called John Harrison who, after many years of research, came up with a design that offered the stability a marine timekeeper needed.
Our concept of time has always been that it is moving forward. We have a future, which will become the past, the interface being the present. Newton and Einstein had their differences, but one fact they agreed on was time could only move forward.
Regarding time travel, there are a number of paradoxes arising from the concepts of travelling backwards or forwards in time. A time traveller would not be able to change the past from the way it is, but could only act in a way that is already consistent with that which has happened. Forward time travel would need the future to be predetermined, but to prevent travel back to the present time would create the paradox explained by backwards time travel and therefore the traveller could never return from the future.
Barry then offered an explanation of Newton’s and Einstein’s theories of gravity.
Newton’s law of universal gravitation defines the attraction between two bodies, and is considered to apply to all objects within the universe. However, Newton could not explain the mechanism of gravity.
Einstein explained the core idea behind relativity. He pondered the consequences of relativity in the context of the speed of light, and formulated a theory of “special relativity” to explain existing phenomena. His theory led to explain the effect of “time dilation”, i.e. the faster you travel through space, the slower you travel through time.
His theory of “special relativity” led him to realise that the effects of acceleration and gravity were indistinguishable, which he termed the “equivalence principle”, which enabled him to apply his knowledge of acceleration to better understand gravity. From there he theorised that the equivalence principle tells us that the effects of gravity and acceleration are indistinguishable, and his theory of “general relativity” predicts everything from the orbits of stars, to the collision of asteroids, to apples falling from a branch to the earth – everything we have come to expect from a theory of gravity.
An application of Einstein’s theories can be seen with the Global Positioning System. GPS, used to pinpoint locations on the Earth’s surface, depends on signals sent from satellites orbiting about 20,000 kilometres above the Earth. If the signals received by those satellites were not compensated for time dilation (Einstein’s “Special Relativity”), after just a day your GPS would misplace your location by about 10 kilometers.
Barry then quoted some measurements used by scientists and physicists, e.g. the definition of a light-year, and their application to some of the vast distances across the universe.
He explained the limitations of our current propulsion systems and why we need to figure out new ways to make things move in space.
Spaceships will need to navigate through space, and will therefore need to measure time in space to determine distance travelled. There is no reference in space to determine time – spaceships will not even be able to determine time from the zodiac in our galaxy.
Scientists have measured the shortest time ever, that is the time it takes a light particle to cross a hydrogen molecule which registered at 247 zeptoseconds i.e. a decimal point followed by 20 zeroes and a 1.
An optical atomic clock has been developed that is so accurate that it will not lose a second over the universe’s entire existence of more than 13 billion years.
There will be a need for compensating time for the warps and ripples of spacetime, as described in Einstein’s “General Relativity”.
In the fast, ever moving, fabric of space, navigation and finding the way back home will be challenging the further we travel from Earth.
One day, when humanity is not limited to a tiny fraction of the speed of light, we might travel to the stars, but so far, faster-than-light travel is possible only in science fiction.
